Practical application of optical devices using silicon as a material of an optical waveguide is important to enable small sizing, large capacity, and low power consumption of optical transmitting and receiving devices. This is because an optical waveguide whose refractive index difference is large can be used, and therefore, it is advantageous for small sizing compared to other materials. Besides, integration with an electronic circuit is easy, and therefore, it is possible to integrate a number of optical transmitting and receiving devices on one chip. Particularly, characteristics of a modulator among optical devices largely affect on the power consumption and size of optical transmitting and receiving devices. In particular, a ring modulator among modulators is advantageous to enable the small sizing and the low power consumption because an element in itself is small, a modulation voltage is small, and an optical loss thereof is small.
However, in a ring modulator, a wavelength band and a modulation efficiency are in a relationship of tradeoff. Accordingly, when high modulation efficiency is to be obtained, the wavelength band is narrow, and it is difficult to match a wavelength of an incident light such as a CW (continuous wave) light and a resonant wavelength. Arts objecting to solve the above-stated problems have been proposed, but it is difficult to enable a stable control.    Patent Literature 1: U.S. Patent Application Laid-open No. 2009/0169149    Patent Literature 2: Japanese Laid-open Patent Publication No. 2009-200091